Monocrystalline silicon is typically the starting material for the fabrication of semiconductor electronic components and solar cells. The monocrystalline silicon is commonly prepared by the Czochralski process, which involves charging polycrystalline silicon into a quartz crucible contained within a crystal pulling chamber. The polycrystalline silicon is melted to create a Si melt. A seed crystal is subsequently immersed into the molten silicon, and a monocrystalline silicon ingot is gradually grown by slow extraction of the ingot from the melt.
During the crystal growth, substantial amounts of silicon-containing impurities, such as gaseous silicon monoxide SiO(g) and silicon vapor Si(g), are typically produced and released into the atmosphere within the pulling chamber. Silicon monoxide SiO(g) is generated as a result of oxygen dissolving from the quartz crucible and reacting with the Si melt, as well as interaction of the crucible with graphite susceptor supports. Because silicon monoxide SiO(g) is unstable, it can readily react with other molecules of SiO(g) produced from the crucible dissolution to produce silicon dioxide SiO2(s) and elemental silicon Si(s). Additionally, because there is a limited amount of oxygen in the chamber, some of the SiO(g) is partially oxidized to form solid SiOx(s) particulates, where x is less than 2.
Formation of solid particulates of SiOx(s), SiO2(s) and elemental silicon Si(s) is a major problem in the crystal pulling industry. The particulates tend to stick and become attached to many parts of the pulling apparatus and exhaust system. The solid particulates can also deposit onto the Si melt if they are not removed, thereby preventing successful monocrystalline growth. Therefore, an inert gas is commonly introduced into the growth chamber to purge the silicon-containing impurities from the growth chamber into the exhaust. Argon gas is typically employed as the inert purge gas, as it is currently the most abundant and the least expensive of the noble gases.
However, while argon is the least expensive of the noble gases, it must be supplied to the crystal growth chamber at high purity levels to avoid contaminating the silicon melt. High-purity argon is generally produced in an air separation plant through a cryogenic distillation process, which is a relatively energy intensive and expensive process. Consequently, the requirement for high-purity argon in the crystal pulling process increases the overall operating costs for the silicon crystal pulling process. High flows of inert gas are required during the purging of impurities from within the crystal pullers, which results in the consumption of large amounts of argon in the crystal pulling apparatus, further adding to the operating costs.
To offset such costs, recycling of the argon gas used in crystal pulling has been attempted. However, the recycling of argon has not proven to be cost effective because the argon effluent from the Si crystal pullers contains impurities such as carbon oxides (CO/CO2), nitrogen (N2), oxygen (O2), hydrogen (H2) and volatile hydrocarbons and the separation process to remove these impurities remains relatively expensive. In particular, the removal of nitrogen and hydrocarbons requires the use of cryogenic distillation since the concentration of nitrogen must be reduced to single-digit ppm levels in the recycled inert gas prior to being re-introduced into the growth chamber for the next cycle. Otherwise, nitrogen can react with the silicon melt to form silicon nitride, which is considered an impurity.
Solid SiOx(s) is unstable and can rapidly react with oxygen in an exothermic reaction to produce substantial heat if the crystal growth chamber and exhaust system are opened to the atmosphere after ingot growth, which is a safety hazard that can cause system damage and/or injury to personnel due to the potential for rapid pressure increase and release of heat. To overcome this danger the industry has typically employed a post-oxidation step which involves backfilling the evacuated crystal puller with air at a slow flow rate. The oxygen in the air slowly reacts with and fully oxidizes the SiOx to SiO2 in a controlled manner, thereby reducing the risk of over-pressurization and release of a substantial amount of heat. However, the post-oxidation step can take up to 24 hours to achieve, thereby adding considerable time to complete the silicon crystal pulling cycle before a new cycle can begin. Such a lag time substantially decreases silicon ingot throughput.
The ability to reduce the post-oxidation period and at the same time reduce the operating and production costs associated with purging impurities from the growth chamber without intentionally increasing impurities in the argon effluent is desirable. Other aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings, and claims appended hereto.